1. Field
The present invention relates to a transmitter receiver apparatus of a time division duplex (TDD) system, which performs transmission and reception in a time division manner.
2. Description of the Related Art
As a transmitter-receiver in the TDD system, there has hitherto been known one having a construction wherein a switch circuit that is in an ON state for a period of a reception slot is provided in a pre-stage of a reception circuit, and a circulator is connected between a transmission/reception antenna, and a transmission power amplifier and a switch circuit. In a technique described in Japanese Laid-Open Patent Publication 05-252074, for a period of a transmission slot, a transmission signal is supplied to the transmission/reception antenna from the power amplifier through the circulator. For the period of the reception slot, a reception signal received by the transmission/reception antenna is supplied to the reception circuit through the circulator and the switch circuit.
However, when performing radio communications using a microwave band or a millimeter wave band, the transmitter-receiver disclosed in the above-described patent document involves the following problems. First, during the transmission period, the switch circuit in the pre-stage of the reception circuit enters an OFF state, and the impedance in the switch circuit during this OFF state has not been taken into consideration, and hence, during the transmission, the impedance as viewed from the transmission side does not become a predetermined load (50Ω). As a result, reflection occurs in the switch circuit during the transmission, thereby raising a problem that the power amplifier on the transmission side, i.e., transmission amplifier cannot be operated in a satisfactory state.
Secondly, in the recent years, with an increasing trend of transmission power, transmission power sneaking into the reception side is on an upward trend. Accordingly, the switch circuit in the pre-stage of the reception circuit is required to have a high ability to block sneaking of the transmission power. It is therefore necessary to constitute the switch circuit by a large size element, resulting in an occurrence of a cost increase problem.